1. Field of the Invention
This invention relates to a method for measuring level of measuring objects and an apparatus thereof, more specifically, a method and an apparatus for improving stability and accuracy of the measurement.
2. Description of the Prior Art
There are various sensors or apparatuses which used for measuring level of measuring objects such as liquid or the like stored in a tank. In these sensors, a capacitance type level sensor is available. The capacitance type level sensor is a sensor which measures level of measuring object such as liquid stored in the tank using measured capacitance.
FIG. 1A shows a diagram illustrating a typical diagram of the capacitance type level sensor which measures liquid 33 stored in the tank 40. The capacitance type level sensor comprises electrodes 10 which has a plurality of electrode 50 in vertical direction to the surface of liquid 33 stored in the tank 40, a power source 2 connected to the electrodes 10 and a controller (not shown).
Capacitance between the tank 40 and the electrodes 50 Is measured with the capacitance type level sensor by applying power supplied from the power supply 2 to the tank 40 and to each electrode 50 provided in the electrodes 10 successively. In other words, the capacitance of a dummy capacitor formed electrically between the tank 40 and the electrode 50 is measured. Value of capacitance between the tank 40 and the electrode 50 is relatively high when the electrode 50 is covered with the liquid 33. On the other hand, value of capacitance between the tank 40 and the electrode 50 is relatively low when the electrode 50 is not covered with the liquid 33.
FIG. 2A shows a relationship between capacitance thus measured and each of the electrodes. According to the graph, center part of a sharp drop LS of the axis of abscissas is detected as the level of the liquid (liquid level).
Also, a typical diagram of another type of the capacitance type level sensor in the prior art is shown in FIG. 1B. The capacitance type level sensor comprises a measuring (long) electrode 50 located in a tank 40, a reference (short) electrode 51 positioned in vicinity of bottom part of the tank 40, a power source 2 connected with both the measuring electrode 50 and the reference electrode 51 and a controller (not shown).
Another type of the capacitance type level sensor is not a level sensor which measure capacitance between each of the electrodes and the tank 40. That is, the principle between these level sensors are different from each other. The capacitance type level sensor measures capacitance between entire of measuring (long) electrode 50 and the tank 40, so that, the level of liquid stored in the tank 40 can be measured directly in accordance with capacitance value. In other words, the level of liquid stored in the tank 40 is measured in accordance with variety of capacitance between the measuring electrode 50 and the tank 40 based on quantity of the liquid. Dielectric constant varies when different kind of liquids 33 are stored in the tank, so that, capacitance measured by the level sensor is changed in accordance with the liquid. Therefore, reference capacitance for each kind of liquids are measured by the reference (short) electrode 51 positioned in vicinity of bottom part of the tank 40, in order to detect level of different kind of liquids. Further, capacitance of the liquid measured by both the measuring electrode and the reference electrode is corrected in accordance with the reference capacitance measured by the reference electrode.
FIG. 2B shows a relationship between capacitance thus measured and level of liquid. The level of liquid is detected in accordance with the relationship.
However, the capacitance type level sensors in the prior art have following problems to resolve. In the capacitance type level sensor shown in FIG. 1A (nonlinear capacitance type level sensor), the level of liquid is detected by measuring the capacitance between the tank 40 and each electrode 50. So that, it is not possible to conduct detection of liquid level accurately, when extraneous objects are adhere on the electrodes.
Also, correction of the capacitance measured by both the measuring electrode and the reference electrode is carried out utilizing the capacitance measured by the reference electrode 51 positioned in vicinity of bottom part of the tank 40. Dielectric constant of the liquid depends on its temperature, even when the liquid stored in the tank is the same kind. So that, there is a temperature difference between a liquid stored in an upper part and a lower part of the tank. In that case, it is not possible to conduct detection of liquid level accurately by correcting the capacitance measured by the reference electrode 51. So that, stability of the level detection can not be maintained. Further, there is a case that the level detection is influenced by moisture contained in a gas accumulated in the upper part of the tank and variation of gas density in the tank.
Still further, the capacitance can not be measured accurately when oil etc. is mixed with the measuring object (liquid) 33 stored in the tank 40 and the oil adheres on the measuring (long) electrode 50, because detected capacitance varies by the oil. In neither of the above described level sensors, level of the measuring object can not be measured when the tank 40 is not formed by conducting materials.